Jet fighter aircraft, in being complex machines which are subject to rigorous demands in operation, occasionally develop minor malfunctions. Some types of these malfunctions can be difficult to detect because the aircraft sometimes contains corrective apparatus which replaces, either actually or by simulation, the malfunctioning component.
An example of simulated replacement is found in the electronic control system of some gas turbine engines which power the aircraft. Such a control system receives inputs from, for example, temperature sensors. If a temperature sensor should fail, thus providing no input or a wild input, the control resorts to a nominal, default, value, which is artificially generated. The artificially generated value in many cases provides such satisfactory operation of the engine that the pilot of the aircraft might not notice a degradation in engine performance despite the failure of the temperature sensor. Thus, apparatus which records and identifies the component failure independent of the pilot is desirable.
Such devices are commonly called fault flags, because they generate a signal ("flag") which indicates the occurrence of the component failure ("fault"). Some fault flags, as used in the art, are not easily readable by aircraft maintenance personnel. Some of them must be connected to external computer circuitry or meters for reading.